- See:
Abnormalities Associated With Vitamin D.
- Discussion:
- primary function of vitamin D is to maintain skeletal calcium homeostasis;
- promotes gut absorption of calcium (and absorption of phosphorous);
- promotes bone absorption;
- important for maintaining adequate quantities of Ca & Phos for bone formation thru its effects on the kidney & gut;
- in addition, it may have a direct effect on bone formation;
- proper functioning of the vitamin D system is necessary for
PTH to maintain plasma calcium effectively, although drops in the
plasma Ca occur only with severe Vit D depletion;
- it appears that only minute amounts of Vit D are necessary for
PTH to carry out its actions on the bone and kidney;
-
biosynthesis:
- provitamin D2 (ergocalciferol from certain plants and animal fat) and 7-dehydroxycholesterol (endogenous) to form cholecalciferol (vit D3);
- provitamin D3 (7-dehydroxycholesterol synthesized in liver & stored in skin);
- in the skin, the provitamin is converted to the active form of cholecalciferol thru the activity of ultraviolet irradiation;
- hepatic hydroxylation to 25-OH vit D3;
- renal hydroxylation to
1,25 di OH D3;
- in proximal renal tubule: conversion to
1,25-dihydroxyvitamin D, 24,25-dihydroxyvitamin D, or 25,26-dihydroxyvitamin D);
-
1,25-dihydroxyvitamin D:
- is the most active of these metabolites;
- this hydroxylation is controled (increased) by
PTH
- 24,25-dihydroxyvitamin D is the least active metabolite;
Labs:
- concentration
of 25-hydroxyvitamin D should remain between 20 - 32 ng per milliliter to
avoid problems;
- references:
-
Review Article: Vitamin D Deficiency
-
Vitamin D Deficiency
- primary effect of lack of Vit D is decrease in miceralization of newly formed bone matrix (osteoid);
- rate of bone formation is decreased;
- net result is less total bone & marked change in quality of bone;
- osteomalacia, group of dz processes characterized by defective mineralizaiton can be caused by any problem resulting in inadequate amounts
of Vit D or low plasma PO4;
- in osteomalacia defective mineralizaiton results in relative increase in amount of osteoid (unmineralized bone matrix) in bone;
- persons who do not receive adequate daily sunlight exposure, such as those confined to home or nursing facility, may be
at special risk for vitamin D deficiency;
- rickets:
- in skeletally immature individuals, mineralization in the growth plates is also affected;
- osteomalacia in this setting is called rickets;
- in rickets, impaired mineralization of cartilage in zone of calcification arrests enchondral ossification & prevents formation of primary spongiosa;
- Dosing:
- Vit D for osteoporosis (see osteoporosis)
- in healthy young adults recommended daily allowance for Vit D is 400 U/day;
- w/ osteoporosis, supplementation with calcium and at least 800 U daily of vitamin D is treatment of choice;
- among elderly patients with hip fractures, 10 to 20 percent have impaired bone mineralization because of vitamin D deficiency;
- l,25-Vitamin D3:
- note that this form of vitamin D, is chiefly indicated for patients with renal failure who cannot synthesis vitamin D3
to the more active 1,25 Vit D3 hormone;
- unlike the standard Vit D3 whose dose is measured in international units, the 1,25 form is measured in micrograms;
- dietary sources of vitamin D3:
- cod liver oil (3.5 oz): 8500 international units
- raw herring, 3.5 oz: 900 international units
- salmon, 3.5 oz: 600 international units
- multivitamin: 400 international units
- milk, 1 cup: 100 international units
- Vit D Toxicity:
- high doses of vitamin D may be toxic;
- toxicity has occurred at levels as low as 2,000 to 5,000 IU / day;
- it is not recommended that anyone consume more than 600 to 800 IU/day without a doctor's recommendation.
Early intervention for postmenopausal osteoporosis.
LB Scheiber and Luis Torregrosa MD. Journal of Musculoskeletal Medicine. May 1999. p 276.
- Vit D Deficiency:
